diff --git a/src/mixed_arena.h b/src/mixed_arena.h
index 9ae19f94d5c..b4822680b6a 100644
--- a/src/mixed_arena.h
+++ b/src/mixed_arena.h
@@ -370,7 +370,7 @@ template<typename SubType, typename T> class ArenaVectorBase {
   void insertAt(size_t index, T item) {
     assert(index <= usedElements); // appending is ok
     resize(usedElements + 1);
-    for (auto i = usedElements; i > index; --i) {
+    for (auto i = usedElements - 1; i > index; --i) {
       data[i] = data[i - 1];
     }
     data[index] = item;